Multijunction metamorphic solar cell

ABSTRACT

A multijunction solar cell including a tandem vertical stack of a least an upper solar subcell, a first middle solar subcell, and bottom solar subcell; a first lateral conduction interlayer disposed adjacent to and beneath the second solar subcell; a blocking p-n diode or insulating layer disposed adjacent to and beneath the first lateral conduction interlayer; and a second lateral conduction layer disposed adjacent to and beneath the blocking p-n diode or insulating layer.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 16/913,149 filed Jan. 26, 2020, which is a divisional of U.S. patent application Ser. No. 15/909,228 filed Mar. 1, 2018, now U.S. Pat. No. 10,700,231, which is a divisional of Ser. No. 15/249,185 filed Aug. 26, 2016, now U.S. Pat. No. 9,985,161, which claims the benefit of U.S. Provisional Application No. 62/288,181 filed Jan. 16, 2016, and Ser. No. 62/243,239 filed Oct. 19, 2015.

The present application is related to U.S. patent application Ser. No. 15/249,204 filed Aug. 26, 2016, now U.S. Pat. No. 9,935,209.

This application is related to U.S. patent application Ser. No. 14/660,092 filed Mar. 17, 2015, which is a division of U.S. patent application Ser. No. 12/716,814 filed Mar. 3, 2010, now U.S. Pat. No. 9,018,521; which was a continuation in part of U.S. patent application Ser. No. 12/337,043 filed Dec. 17, 2008.

This application is also related to U.S. patent application Ser. No. 13/872,663 filed Apr. 29, 2012, now U.S. Pat. No. 10,541,349, which was also a continuation-in-part of application Ser. No. 12/337,043, filed Dec. 17, 2008.

This application is also related to U.S. patent application Ser. Nos. 14/828,197 and 14/828,206 filed Aug. 17, 2015.

All of the above related applications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to solar cells and the fabrication of solar cells, and more particularly the design and specification of a multijunction solar cell using electrically coupled but spatially separated semiconductor bodies based on III-V semiconductor compounds.

Description of the Related Art

Solar power from photovoltaic cells, also called solar cells, has been predominantly provided by silicon semiconductor technology. In the past several years, however, high-volume manufacturing of III-V compound semiconductor multijunction solar cells for space applications has accelerated the development of such technology not only for use in space but also for terrestrial solar power applications. Compared to silicon, III-V compound semiconductor multijunction devices have greater energy conversion efficiencies and generally more radiation resistance, although they tend to be more complex to properly specify and manufacture. Typical commercial III-V compound semiconductor multijunction solar cells have energy efficiencies that exceed 27% under one sun, air mass 0 (AM0), illumination, whereas even the most efficient silicon technologies generally reach only about 18% efficiency under comparable conditions. The higher conversion efficiency of III-V compound semiconductor solar cells compared to silicon solar cells is in part based on the ability to achieve spectral splitting of the incident radiation through the use of a plurality of photovoltaic regions with different band gap energies, and accumulating the current from each of the regions.

In satellite and other space related applications, the size, mass and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as payloads become more sophisticated, and applications anticipated for five, ten, twenty or more years, the power-to-weight ratio and lifetime efficiency of a solar cell becomes increasingly more important, and there is increasing interest not only the amount of power provided at initial deployment, but over the entire service life of the satellite system, or in terms of a design specification, the amount of power provided at the “end of life” (EOL).

The efficiency of energy conversion, which converts solar energy (or photons) to electrical energy, depends on various factors such as the design of solar cell structures, the choice of semiconductor materials, and the thickness of each cell. In short, the energy conversion efficiency for each solar cell is dependent on the optimum utilization of the available sunlight across the solar spectrum as well as the “age” of the solar cell, i.e. the length of time it has been deployed and subject to degradation associated with the space environment. As such, the characteristic of sunlight absorption in semiconductor material, also known as photovoltaic properties, is critical to determine the most efficient semiconductor to achieve the optimum energy conversion to meet customer requirements.

Typical III-V compound semiconductor solar cells are fabricated on a semiconductor wafer in vertical, multijunction structures or stacked sequence of solar subcells, each subcell formed with appropriate semiconductor layers and including a p-n photoactive junction. Each subcell is designed to convert photons over different spectral or wavelength bands to electrical current. After the sunlight impinges on the front of the solar cell, and photons pass through the subcells, the photons in a wavelength band that are not absorbed and converted to electrical energy in the region of one subcell propagate to the next subcell, where such photons are intended to be captured and converted to electrical energy, assuming the downstream subcell is designed for the photon's particular wavelength or energy band.

The individual solar cells or wafers are then disposed in horizontal arrays or panels, with the individual solar cells connected together in an electrical series and/or parallel circuit. The shape and structure of an array, as well as the number of cells it contains, are determined in part by the desired output voltage and current.

The energy conversion efficiency of multijunction solar cells is affected by such factors as the number of subcells, the thickness of each subcell, the composition and doping of each active layer in a subcell, and the consequential band structure, electron energy levels, conduction, and absorption of each subcell, as well as its exposure to radiation in the ambient environment over time. Factors such as the short circuit current density (J_(sc)), the open circuit voltage (V_(oc)), and the fill factor are also important. Another parameter of consideration is the difference between the band gap and the open circuit voltage, or (E_(g)−V_(oc)), of a particular active layer, and such parameters may vary over time (i.e. during the operational life of the system). Accordingly, such parameters are NOT simple “result effective” variables (as discussed and emphasized below) to those skilled in the art confronted with complex design specifications and practical operational considerations.

One of the important mechanical or structural considerations in the choice of semiconductor layers for a solar cell is the desirability of the adjacent layers of semiconductor materials in the solar cell, i.e. each layer of crystalline semiconductor material that is deposited and grown to form a solar subcell, have similar crystal lattice constants or parameters. The present application is directed to solar cells with several substantially lattice matched subcells, but including at least one subcell which is lattice mismatched, and in a particular embodiment to a five junction (5J) solar cell using electrically coupled but spatially separated four junction (4J) semiconductor bodies based on III-V semiconductor compounds.

SUMMARY OF THE DISCLOSURE Objects of the Disclosure

It is an object of the present disclosure to provide increased photoconversion efficiency in a multijunction solar cell for space applications over the operational life of the photovoltaic power system.

It is another object of the present disclosure to provide in a multijunction solar cell in which the selection of the composition of the subcells and their band gaps maximizes the efficiency of the solar cell at a predetermined high temperature (in the range of 40 to 100 degrees Centigrade) in deployment in space at AM0 at a predetermined time after the initial deployment, such time being at least one year, and in the range of one to twenty-five years.

It is another object of the present disclosure to provide a four junction solar cell subassembly in which the average band gap of all four cells in the subassembly is greater than 1.44 eV, and to couple the subassembly in electrical series with at least one additional subcell in an adjacent solar cell subassembly.

It is another object of the present disclosure to provide a lattice mis-matched five junction solar cell in which the bottom subcell is intentionally designed to have a short circuit current that is substantially greater than current through the top three subcells when measured at the “beginning-of-life” or time of initial deployment.

It is another object of the present disclosure to provide a five-junction (5J) solar assembly assembled from two four-junction (4J) solar cell subassemblies so that the total current provided by the two subassemblies matches the total current handling capability of the bottom subcell of the assembly.

It is another object of the present disclosure to match the larger short circuit current of the bottom subcell of the solar cell assembly with two or three parallel stacks of solar subcells, i.e. a configuration in which the value of the short circuit current of the bottom subcell is at least twice, or at least three times, that of the solar subcells in each parallel stack which are connected in a series with the bottom subcell. Stated another way, given the choice of the composition of the bottom subcell, and there by the short circuit current of the bottom subcell, it is an object of the disclosure that the upper subcell stack be specified and designed to have a short circuit which is one-half or less than that of the bottom subcell.

Some implementations of the present disclosure may incorporate or implement fewer of the aspects and features noted in the foregoing objects.

Features of the Invention

Briefly, and in general terms, the present disclosure describes solar cells that include a solar cell assembly of two or more solar cell subassemblies, each of which includes a respective monolithic semiconductor body composed of a tandem stack of solar subcells, where the subassemblies are interconnected electrically to one another.

As described in greater detail, the inventors of the present application have discovered that interconnecting two or more spatially split multi-junction solar cell subassemblies can be advantageous. The spatial split can be provided for multiple solar cell subassemblies monolithically formed on a single substrate. Alternatively, the solar cell subassemblies can be fabricated as separate semiconductor chips that can be coupled together electrically.

One advantage of interconnecting two or more spatially split multi-junction solar cell subassemblies is that such an arrangement can allow accumulation of the subcells in relatively few semiconductor bodies.

Further, selection of relatively high band gap semiconductor materials for the top subcells can provide for increased photoconversion efficiency in a multijunction solar cell for outer space or other applications over the operational life of the photovoltaic power system. For example, increased photoconversion efficiency at a predetermined time after initial deployment of the solar cell can be achieved.

Thus, in one aspect, a monolithic solar cell subassembly includes a first semiconductor body including an upper first solar subcell composed of (aluminum) indium gallium phosphide ((Al)InGaP); a second solar subcell disposed adjacent to and lattice matched to said upper first subcell, the second solar subcell composed of (aluminum) gallium arsenide ((Al)GaAs) or indium gallium arsenide phosphide (InGaAsP); and a bottom subcell lattice matched to said second subcell and composed of indium gallium arsenide (In)GaAs.

The aluminum (or Al) constituent element, or indium (or In), shown in parenthesis in the preceding formula means that Al or In (as the case may be) is an optional constituent, and in the case of Al, in this instance may be used in an amount ranging from 0% to 30%. The subcells are configured so that the current density of the upper first subcell and the second subcell have a substantially equal predetermined first value, and the current density of the bottom subcell is at least twice that of the predetermined first value.

In another aspect, the present disclosure provides a four or five junction solar cell assembly including a terminal of first polarity and a terminal of second polarity comprising: a first semiconductor body including a tandem vertical stack of at least a first upper solar subcell, a second solar subcell, and a bottom solar subcell, the first upper subcell having a top contact connected to the terminal of first polarity, and the bottom solar subcell having a top contact and a bottom contact; a second semiconductor body disposed adjacent to the first semiconductor body and including a tandem vertical stack of at least a first upper, a second and a bottom solar subcells, the first upper subcell of the second semiconductor body having a top contact connected to the terminal of first polarity, and the bottom subcell of the second semiconductor body having a top contact and a bottom contact with its bottom contact connected to the terminal of second polarity; wherein the first and second semiconductor body each comprises a first highly doped lateral conduction layer electrically connected to each other and disposed adjacent to and beneath the second solar subcell of each respective body, the first lateral conduction layer being compositionally graded so as to lattice match the lattice constant of the second solar subcell at the top, and the lattice constant of the bottom subcell of each respective body at the bottom, and wherein the first and second semiconductor body each comprises a blocking p-n diode or insulating layer disposed adjacent to and beneath the first highly doped lateral conduction layer; and a second highly doped lateral conduction layer disposed adjacent to and beneath the respective blocking p-n diode or insulating layer, an electrical connection between the first lateral conduction layer of the first and the second semiconductor body and the second lateral conduction layer of the first semiconductor body; the bottom solar subcell of each respective body being disposed adjacent to and beneath the second highly doped lateral conduction layer, with the bottom subcell of the first semiconductor body being connected in a series electrical circuit with the bottom subcell of the second semiconductor body so that at least a four junction solar cell is formed by the assembly.

In some embodiments, there further comprises a first electrical contact on the first lateral conduction layer, and a second electrical contact on the second lateral conduction layer, and wherein the electrical connection is a wire welded to the first electrical contact at one end, and the second electrical contact at the other end.

In some embodiments, there further comprises an electrical connection between the bottom contact of the first bottom subcell, and the top contact of the second bottom subcell.

In some embodiments, the bottom subcell of the first and second semiconductor bodies has a band gap in the range of approximately 0.67 eV, the second subcell of the first and second semiconductor bodies has a band gap in the range of approximately 1.3 to 1.5 eV and the upper first subcell of the first and second semiconductor bodies has a band gap in the range of 1.8 to 2.0 eV.

In some embodiments, the first and second semiconductor bodies include a tandem vertical stack of at least a first upper, a second and third solar subcells, and the first and second bottom subcells, so that at least a five junction solar cell is formed by the assembly, with the first upper, a second and third solar subcells being current matched, and the first and second bottom subcells being current mismatched from the first upper, a second and third solar subcells.

In some embodiments, the bottom subcell of the first and second semiconductor bodies has a band gap in the range of approximately 0.67 eV, the third subcell of the first and second semiconductor bodies has a band gap in the range of approximately 1.41 eV and 1.31 eV, the second subcell of the first and second semiconductor bodies has a band gap in the range of approximately 1.65 to 1.8 eV and the upper first subcell of the first and second semiconductor bodies has a band gap in the range of 2.0 to 2.20 eV.

In some embodiments, the second semiconductor body further comprises a first highly doped lateral conduction layer disposed adjacent to and beneath the second solar subcell, the first lateral conduction layer being compositionally graded so as to lattice match the lattice constant of the second solar subcell at the top, and the lattice constant of the fourth bottom subcell at the bottom.

In some embodiments, the upper first subcell is composed of indium gallium aluminum phosphide (InGaAlP); the second solar subcell includes an emitter layer composed of indium gallium phosphide (InGaP), indium aluminum gallium arsenide ((In)AlGaAs) or indium gallium arsenide phosphide (InGaAsP), and a base layer composed of indium aluminum gallium arsenide or indium gallium arsenide phosphide (InGaAsP); the third solar subcell is composed of indium gallium arsenide; the fourth subcell is composed of germanium or SiGeSn, GaSb, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN. InGaSbBiN; and the graded interlayer is composed of In_(x)Ga_(1-x)As or In_(x)Ga_(1-x)P with 0<x<1, 0<y<1, and x and y selected such that the band gap is in the range of 1.41 eV to 1.6 eV and may vary throughout its thickness.

In some embodiments, the upper first subcell has a band gap in the range of 2.0 to 2.20 eV and is composed of indium gallium aluminum phosphide (InGaAlP); the second solar subcell has a band gap in the range of approximately 1.65 to 1.8 eV and includes an emitter layer composed of indium gallium phosphide (InGaP), indium aluminum gallium arsenide ((In)AlGaAs) or indium gallium arsenide phosphide (InGaAsP), and a base layer composed of indium aluminum gallium arsenide or indium gallium arsenide phosphide (InGaAsP); the third solar subcell has a band gap in the range of approximately 1.3 to 1.41 eV and is composed of indium gallium arsenide; and the fourth subcell is composed of germanium or SiGeSn, GaSb, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN. InGaSbBiN; the upper first subcell is composed of indium gallium aluminum phosphide.

In some embodiments, there further comprises a distributed Bragg reflector (DBR) layer disposed above the bottom solar subcell and arranged so that light can enter and pass through the solar subcell disposed above the bottom subcell and at least a portion of which can be reflected back into the solar subcell located above the bottom subcell by the DBR layer, and the distributed Bragg reflector layer is composed of a plurality of alternating layers of lattice matched materials with discontinuities in their respective indices of refraction.

In some embodiments, the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength, and the DBR layer includes a first DBR layer composed of a plurality of p type Al_(x)Ga_(1-x)(In)As layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type Al_(y)Ga_(1-y)(In)As layers, where y is greater than x, and the term (In) denotes an optional inclusion of up to 10% indium.

In some embodiments, there further comprises a distributed Bragg reflector (DBR) layer adjacent to and between the third and the fourth solar subcells and arranged so that light can enter and pass through the third solar subcell and at least a portion of which can be reflected back into the third solar subcell by the DBR layer, and the distributed Bragg reflector layer is composed of a plurality of alternating layers of lattice matched materials with discontinuities in their respective indices of refraction.

In some embodiments, the first and second semiconductor bodies constitute a single semiconductor body that has been singulated to form to spatially separated bodies.

In some embodiments, the bottom subcell is comprised of a direct or indirect band gap material such that the lowest direct band gap of the material is greater than 0.75 eV, and the average band gap of all four subcells in each of the semiconductor bodies (i.e., the sum of the four lowest direct or indirect band gaps of the materials of each subcell in the semiconductor body divided by 4) is greater than 1.44 eV.

In some embodiments, there further comprises a first alpha layer deposited over the first lateral conduction layer to a thickness of between 0.25 and 1.0 micron to prevent threading dislocations from propagating, either opposite to the direction of growth or in the direction of growth into the second subcell.

In some embodiments, there further comprises a second alpha layer deposited over the blocking p-n diode or insulating layer to a thickness of between 0.25 and 1.0 micron to prevent threading dislocations from propagating, either opposite to the direction of growth or in the direction of growth into the second subcell.

In some embodiments, there further comprises a tunnel diode disposed over the first lateral conduction layer and below the distributed Bragg reflector (DBR) layer.

In another aspect, the present disclosure provides a multijunction solar cell assembly including a terminal of first polarity and a terminal of second polarity comprising: a first semiconductor body including a tandem vertical stack of at least a first upper solar subcell, a second solar subcell, and a bottom solar subcell, the first upper subcell having a top contact connected to the terminal of first polarity, and the bottom solar subcell having a top contact and a bottom contact; a second semiconductor body disposed adjacent to the first semiconductor body and including a tandem vertical stack of at least a first upper, a second and a bottom solar subcells, the first upper subcell of the second semiconductor body having a top contact connected to the terminal of first polarity, and the bottom subcell of the second semiconductor body having its bottom contact connected to the terminal of second polarity; wherein the first and second semiconductor body each comprises a conductive graded interlayer disposed adjacent to and beneath the second solar subcell of each respective body, the graded interlayer being compositionally graded so as to lattice match the lattice constant of the second solar subcell at the top, and the lattice constant of the bottom subcell of each respective body at the bottom, and wherein the first and second semiconductor body each comprises a blocking p-n diode or insulating layer disposed adjacent to and beneath the graded interlayer; and a highly doped lateral conduction layer disposed adjacent to and beneath the respective blocking p-n diode or insulating layer, an electrical connection between the graded interlayer of the first and the second semiconductor body and the lateral conduction layer of the first semiconductor body; the bottom solar subcell of each respective body being disposed adjacent to and beneath the lateral conduction layer, with the bottom contact of the bottom subcell of the first semiconductor body being connected in a series electrical circuit with the top contact of the bottom subcell of the second semiconductor body so that at least a four junction solar cell is formed by the assembly.

In another aspect, the present disclosure provides a method of forming a solar cell module including a terminal of first polarity and a terminal of second polarity comprising: forming a first semiconductor body including a tandem vertical stack of at least a first upper, a second, third and fourth solar subcells which are current matched, the first upper subcell having a top contact connected to the terminal of first polarity and a bottom fourth solar subcell that is current mismatched from the first, second and third solar subcells; forming a second semiconductor body including a tandem vertical stack of at least a first upper, second and third subcells, and a bottom fourth solar subcell that is current mismatched from the first, second and third solar subcells; disposing the first semiconductor body adjacent to the second semiconductor body and connecting the top contact of the first upper subcells of the first and second semiconductor bodies; and connecting the fourth subcell of the first semiconductor body in a series electrical circuit with the fourth subcell of the second semiconductor body, wherein the first and second semiconductor body each comprises a first highly doped lateral conduction layer electrically connected to each other and disposed adjacent to and beneath the second solar subcell of each respective body, the first lateral conduction layer being compositionally graded so as to lattice match the lattice constant of the second solar subcell at the top, and the lattice constant of the bottom subcell of each respective body at the bottom, and wherein the first and second semiconductor body each comprises a blocking p-n diode or insulating layer disposed adjacent to and beneath the first highly doped lateral conduction layer; and a second highly doped lateral conduction layer disposed adjacent to and beneath the respective blocking p-n diode or insulating layer, an electrical connection between the first lateral conduction layer of the second semiconductor body and the second lateral conduction layer of the first semiconductor body; the bottom solar subcell of each respective body being disposed adjacent to and beneath the second highly doped lateral conduction layer, with the bottom subcell of the first semiconductor body being connected in a series electrical circuit with the bottom subcell of the second semiconductor body so that at least a four junction solar cell is formed by the assembly.

In some implementations, the average band gap of all of the subcells in each semiconductor body is greater than 1.44 eV. In some instances, the band gap of the first upper subcell is in the range of 2.0 to 2.20 eV, the band gap of the second subcell is in the range of 1.65 to 1.8 eV, the third subcell has a band gap of approximately 1.41 eV, and the band gap of the bottom subcell is in the range of 0.6 to 0.8 eV. Other implementations may have different band gap ranges.

Some implementations can include additional solar subcells.

The solar cell subassembly can further include a plurality of openings in the first semiconductor body, each of the openings extending from a top surface of the first semiconductor body to a different respective contact layer in the first semiconductor body. Thus, for example, a first opening in the first semiconductor body can extend from the top surface of the semiconductor body to the first lateral conduction layer. A metallic contact pad can be disposed on the lateral conduction layer. A second opening in the first semiconductor body can extend from the top surface of the semiconductor body to the contact back metal later of the bottom subcell.

In some implementations, the short circuit density (J_(sc)/cm²) of the (Al)InGaP first upper subcell is approximately 12 mA/cm². The short circuit density (J_(sc)/cm²) of the first upper subcell may have another value for different implementations.

In another aspect, a solar cell assembly includes a terminal of first polarity and a terminal of second polarity. The solar cell assembly includes a first semiconductor body including a tandem vertical stock of at least a first upper, a second, a third and a fourth solar subcell, the first upper subcell having a top contact connected to the terminal of first polarity. The solar cell assembly further includes a second semiconductor body disposed adjacent to the first semiconductor body and including a tandem vertical stock of at least a first upper, a second, third and a fourth bottom solar subcells, the fourth bottom subcell having a bottom contact connected to the terminal of second polarity. The fourth subcell of the first semiconductor body is connected in a series electrical circuit with the fourth subcell of the second semiconductor body.

In some cases (e.g., for an assembly having two subassemblies), the short circuit density (J_(sc)/cm²) of each of the first and second subcells is approximately 12 mA/cm². In other instances (e.g., for an assembly having three subassemblies), the short circuit density (J_(sc)/cm²) of each of the first, second and third middle subcells is approximately 10 mA/cm². The short circuit density (J_(sc)/cm²) of the bottom subcell in the foregoing cases can be approximately greater than 24 mA/cm². However, the short circuit densities (J_(sc)/cm²) may have different values in some implementations.

In some embodiments, the fourth subcell is germanium.

In some embodiments, the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN. InGaSbBiN.

In some embodiments, the second subcell has a band gap of approximately 1.73 eV and the upper first subcell has a band gap of approximately 2.10 eV.

In some embodiments, the upper first subcell is composed of indium gallium aluminum phosphide; the second solar subcell includes an emitter layer composed of indium gallium phosphide or indium aluminum gallium arsenide, and a base layer composed of indium aluminum gallium arsenide; and the third solar subcell is composed of indium gallium arsenide.

In some embodiments, the upper first subcell is composed of an indium gallium aluminum phosphide; the second solar subcell includes an emitter layer composed of indium gallium phosphide or indium aluminum gallium arsenide, and a base layer composed of indium aluminum gallium arsenide; the third solar subcell is composed of indium gallium arsenide; the fourth subcell is composed of germanium; and the graded lateral conduction layer is composed of In_(x)Ga_(1-x)As with 0<x<1, 0<y<1, and x and y selected such that the band gap varies throughout its thickness.

In some embodiments, the selection of the composition of the subcells and their band gaps maximizes the efficiency at high temperature (in the range of 40 to 70 degrees Centigrade) in deployment in space at a predetermined time after the initial deployment (referred to as the beginning-of-life or (BOL), such predetermined time being referred to as the end-of-life (EOL), and the average band gap of all four cells greater than 1.44 eV.

In another aspect, the present disclosure provides a four junction solar cell comprising an upper first solar subcell composed of a semiconductor material having a first band gap; a second solar subcell adjacent to said first solar subcell and composed of a semiconductor material having a second band gap smaller than the first band gap and being lattice matched with the upper first solar subcell; a third solar subcell adjacent to said second solar subcell and composed of a semiconductor material having a third band gap smaller than the second band gap and being lattice matched with the second solar subcell; and a fourth solar subcell adjacent to and lattice mismatched to said third solar subcell and composed of a semiconductor material having a fourth band gap smaller than the third band gap; wherein the average band gap of all four subcells (i.e., the sum of the four band gaps of each subcell divided by four) is greater than 1.44 eV.

In another aspect, the present disclosure provides a method of manufacturing a four junction solar cell comprising providing a substrate; growing on the substrate a sequence of layers of semiconductor material using a semiconductor disposition process to form a solar cell comprising a plurality of subcells including a metamorphic layer, a third subcell disposed over the metamorphic layer and having a band gap of approximately 1.41 eV, a second subcell disposed over the third subcell and having a band gap in the range of approximately 1.65 to 1.8 eV and an upper first subcell disposed over the second subcell and having a band gap in the range of 2.0 to 2.20 eV.

In some embodiments, there further comprises a back surface field (BSF) layer disposed directly adjacent to the bottom surface of the third subcell, and at least one distributed Bragg reflector (DBR) layer directly below the BSF layer.

In some embodiments, additional layer(s) may be added or deleted in the cell structure without departing from the scope of the present disclosure.

Some implementations of the present disclosure may incorporate or implement fewer of the aspects and features noted in the foregoing summaries.

Additional aspects, advantages, and novel features of the present disclosure will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the disclosure. While the disclosure is described below with reference to preferred embodiments, it should be understood that the disclosure is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments in other fields, which are within the scope of the disclosure as disclosed and claimed herein and with respect to which the disclosure could be of utility.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a graph representing the BOL value of the parameter E_(g)/q−V_(oc) at 28° C. plotted against the band gap of certain ternary and quaternary materials defined along the x-axis;

FIG. 2 is a cross-sectional view of a first embodiment of a first semiconductor body including a four junction solar cell after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer and etching contact steps on lower levels according to the present disclosure;

FIG. 3 is a cross-sectional view of a first embodiment of a second semiconductor body including a four junction solar cell after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer, according to the present disclosure;

FIG. 4 is a cross-sectional view of an embodiment of a five junction solar cell after following electrical connection of the first and second semiconductor bodies according to the present disclosure; and

FIG. 5 is a schematic diagram of the five junction solar cell of FIG. 5.

GLOSSARY OF TERMS

“III-V compound semiconductor” refers to a compound semiconductor formed using at least one elements from group III of the periodic table and at least one element from group V of the periodic table. III-V compound semiconductors include binary, tertiary and quaternary compounds. Group III includes boron (B), aluminum (Al), gallium (Ga), indium (In) and thallium (T). Group V includes nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb) and bismuth (Bi).

“Band gap” refers to an energy difference (e.g., in electron volts (eV)) separating the top of the valence band and the bottom of the conduction band of a semiconductor material.

“Beginning of Life (BOL)” refers to the time at which a photovoltaic power system is initially deployed in operation.

“Bottom subcell” refers to the subcell in a multijunction solar cell which is furthest from the primary light source for the solar cell.

“Compound semiconductor” refers to a semiconductor formed using two or more chemical elements.

“Current density” refers to the short circuit current density J_(sc) through a solar subcell through a given planar area, or volume, of semiconductor material constituting the solar subcell.

“Deposited”, with respect to a layer of semiconductor material, refers to a layer of material which is epitaxially grown over another semiconductor layer.

“End of Life (EOL)” refers to a predetermined time or times after the Beginning of Life, during which the photovoltaic power system has been deployed and has been operational. The EOL time or times may, for example, be specified by the customer as part of the required technical performance specifications of the photovoltaic power system to allow the solar cell designer to define the solar cell subcells and sublayer compositions of the solar cell to meet the technical performance requirement at the specified time or times, in addition to other design objectives. The terminology “EOL” is not meant to suggest that the photovoltaic power system is not operational or does not produce power after the EOL time.

“Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.

“Inverted metamorphic multijunction solar cell” or “IMM solar cell” refers to a solar cell in which the subcells are deposited or grown on a substrate in a “reverse” sequence such that the higher band gap subcells, which would normally be the “top” subcells facing the solar radiation in the final deployment configuration, are deposited or grown on a growth substrate prior to depositing or growing the lower band gap subcells.

“Layer” refers to a relatively planar sheet or thickness of semiconductor or other material.

The layer may be deposited or grown, e.g., by epitaxial or other techniques.

“Lattice mismatched” refers to two adjacently disposed materials or layers (with thicknesses of greater than 100 nm) having in—plane lattice constants of the materials in their fully relaxed state differing from one another by less than 0.02% in lattice constant. (Applicant expressly adopts this definition for the purpose of this disclosure, and notes that this definition is considerably more stringent than that proposed, for example, in U.S. Pat. No. 8,962,993, which suggests less than 0.6% lattice constant difference).

“Metamorphic layer” or “graded interlayer” refers to a layer that achieves a gradual transition in lattice constant generally throughout its thickness in a semiconductor structure.

“Middle subcell” refers to a subcell in a multijunction solar cell which is neither a Top Subcell (as defined herein) nor a Bottom Subcell (as defined herein).

“Short circuit current (I_(sc))” refers to the amount of electrical current through a solar cell or solar subcell when the voltage across the solar cell is zero volts, as represented and measured, for example, in units of milliamps.

“Short circuit current density”—see “current density”.

“Solar cell” refers to an electronic device operable to convert the energy of light directly into electricity by the photovoltaic effect.

“Solar cell assembly” refers to two or more solar cell subassemblies interconnected electrically with one another.

“Solar cell subassembly” refers to a stacked sequence of layers including one or more solar subcells.

“Solar subcell” refers to a stacked sequence of layers including a p-n photoactive junction composed of semiconductor materials. A solar subcell is designed to convert photons over different spectral or wavelength bands to electrical current.

“Substantially current matched” refers to the short circuit current through adjacent solar subcells being substantially identical (i.e. within plus or minus 1%).

“Top subcell” or “upper subcell” refers to the subcell in a multijunction solar cell which is closest to the primary light source for the solar cell.

“ZTJ” refers to the product designation of a commercially available SolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of the present invention will now be described including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of the actual embodiment nor the relative dimensions of the depicted elements, and are not drawn to scale.

A variety of different features of multijunction solar cells (as well as inverted metamorphic multijunction solar cells) are disclosed in the related applications noted above. Some, many or all of such features may be included in the structures and processes associated with the non-inverted or “upright” solar cells of the present disclosure. However, more particularly, the present disclosure is directed to the fabrication of a multijunction lattice matched solar cell grown over a metamorphic layer which is grown on a single growth substrate which forms a solar cell subassembly. More specifically, however, in some embodiments, the present disclosure relates to multijunction solar cell subassemblies with direct band gaps in the range of 2.0 to 2.15 eV (or higher) for the top subcell, and (i) 1.65 to 1.8 eV, and (ii) 1.41 eV for the middle subcells, and 0.6 to 0.8 eV direct or indirect band gaps, for the bottom subcell, respectively, and the connection of two or more such subassemblies to form a solar cell assembly.

As described in greater detail, the present application notes that interconnecting two or more spatially split multijunction solar cell subassemblies can be advantageous. The spatial split can be provided for multiple solar cell subassemblies monolithically formed on the same substrate. Alternatively, the solar cell subassemblies can be fabricated as separate semiconductor chips that can be coupled together electrically.

In general terms, a solar cell assembly in accordance with one aspect of the present disclosure can include a terminal of first polarity and a terminal of second polarity. The solar cell assembly includes a first semiconductor subassembly including a tandem vertical stack of at least a first upper, a second, third and fourth bottom solar subcells, the first upper subcell having a top contact connected to the terminal of first polarity. A second semiconductor subassembly is disposed adjacent to the first semiconductor subassembly and includes a tandem vertical stock of at least a first upper, a second, third, and fourth bottom solar subcells, the fourth bottom subcell having a bask side contact connected to the terminal of second polarity. The fourth subcell of the first semiconductor subassembly is connected in a series electrical circuit with the third subcell of the second semiconductor subassembly. Thus, a five-junction solar assembly is assembled from two four-junction solar cell subassemblies.

In some cases, the foregoing solar cell assembly can provide increased photoconversion efficiency in a multijunction solar cell for outer space or other applications over the operational life of the photovoltaic power system.

Another aspect of the present disclosure is that to provide a four junction solar cell assembly composed of spatially separated solar cell subassemblies, the average band gap of all four subcells (i.e., the sum of the four band gaps of each subcell divided by 4) in each solar cell subassembly being greater than 1.44 eV.

Another descriptive aspect of the present disclosure is to characterize the fourth subcell as being composed of an indirect or direct band gap material such that the lowest direct band gap is greater than 0.75 eV, in some embodiments.

In some embodiments, the fourth subcell in each solar cell subassembly is germanium, while in other embodiments the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN. InGaSbBiN or other III-V or II-VI compound semiconductor material.

The indirect band gap of germanium at room temperature is about 0.66 eV, while the direct band gap of germanium at room temperature is 0.8 eV. Those skilled in the art will normally refer to the “band gap” of germanium as 0.66 eV, since it is lower than the direct band gap value of 0.8 eV.

The recitation that “the fourth subcell has a direct band gap of greater than 0.75 eV” is therefore expressly meant to include germanium as a possible semiconductor for the fourth subcell, although other semiconductor materials can be used as well.

More specifically, the present disclosure intends to provide a relatively simple and reproducible technique that does not employ inverted processing associated with inverted metamorphic multijunction solar cells, and is suitable for use in a high volume production environment in which various semiconductor layers are grown on a growth substrate in an MOCVD reactor, and subsequent processing steps are defined and selected to minimize any physical damage to the quality of the deposited layers, thereby ensuring a relatively high yield of operable solar cells meeting specifications at the conclusion of the fabrication processes.

Prior to discussing the specific embodiments of the present disclosure, a brief discussion of some of the issues associated with the design of multijunction solar cells, and in particular metamorphic solar cells, and the context of the composition or deposition of various specific layers in embodiments of the product as specified and defined by Applicant is in order.

There are a multitude of properties that should be considered in specifying and selecting the composition of, inter alia, a specific semiconductor layer, the back metal layer, the adhesive or bonding material, or the composition of the supporting material for mounting a solar cell thereon. For example, some of the properties that should be considered when selecting a particular layer or material are electrical properties (e.g. conductivity), optical properties (e.g., band gap, absorbance and reflectance), structural properties (e.g., thickness, strength, flexibility, Young's modulus, etc.), chemical properties (e.g., growth rates, the “sticking coefficient” or ability of one layer to adhere to another, stability of dopants and constituent materials with respect to adjacent layers and subsequent processes, etc.), thermal properties (e.g., thermal stability under temperature changes, coefficient of thermal expansion), and manufacturability (e.g., availability of materials, process complexity, process variability and tolerances, reproducibility of results over high volume, reliability and quality control issues).

In view of the trade-offs among these properties, it is not always evident that the selection of a material based on one of its characteristic properties is always or typically “the best” or “optimum” from a commercial standpoint or for Applicant's purposes. For example, theoretical studies may suggest the use of a quaternary material with a certain band gap for a particular subcell would be the optimum choice for that subcell layer based on fundamental semiconductor physics. As an example, the teachings of academic papers and related proposals for the design of very high efficiency (over 40%) solar cells may therefore suggest that a solar cell designer specify the use of a quaternary material (e.g., InGaAsP) for the active layer of a subcell. A few such devices may actually be fabricated by other researchers, efficiency measurements made, and the results published as an example of the ability of such researchers to advance the progress of science by increasing the demonstrated efficiency of a compound semiconductor multijunction solar cell. Although such experiments and publications are of “academic” interest, from the practical perspective of the Applicants in designing a compound semiconductor multijunction solar cell to be produced in high volume at reasonable cost and subject to manufacturing tolerances and variability inherent in the production processes and suited for specific applications such as the space environment where the efficiency over the entire operational life is an important goal, such an “optimum” design from an academic perspective is not necessarily the most desirable design in practice, and the teachings of such studies more likely than not point in the wrong direction and lead away from the proper design direction. Stated another way, such references may actually “teach away” from Applicant's research efforts and the ultimate solar cell design proposed by the Applicants.

The lattice constants and electrical properties of the layers in the semiconductor structure are preferably controlled by specification of appropriate reactor growth temperatures and times, and by use of appropriate chemical composition and dopants. The use of a deposition method, such as Molecular Beam Epitaxy (MBE), Organo Metallic Vapor Phase Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD), or other vapor deposition methods for the growth may enable the layers in the monolithic semiconductor structure forming the cell to be grown with the required thickness, elemental composition, dopant concentration and grading and conductivity type.

The present disclosure is directed to, in one embodiment, a growth process using a metal organic chemical vapor deposition (MOCVD) process in a standard, commercially available reactor suitable for high volume production. More particularly, the present disclosure is directed to the materials and fabrication steps that are particularly suitable for producing commercially viable multijunction solar cells using commercially available equipment and established high-volume fabrication processes, as contrasted with merely academic expositions of laboratory or experimental results.

In view of the foregoing, it is further evident that the identification of one particular constituent element (e.g. indium, or aluminum) in a particular subcell, or the thickness, band gap, doping, or other characteristic of the incorporation of that material in a particular subcell, is not a “result effective variable” that one skilled in the art can simply specify and incrementally adjust to a particular level and thereby increase the efficiency of a solar cell at the beginning of life or the end of life. The efficiency of a solar cell is not a simple linear algebraic equation as a function of the amount of gallium or aluminum or other element in a particular layer. The growth of each of the epitaxial layers of a solar cell in an MOCVD reactor is a non-equilibrium thermodynamic process with dynamically changing spatial and temporal boundary conditions that is not readily or predictably modeled. The formulation and solution of the relevant simultaneous partial differential equations covering such processes are not within the ambit of those of ordinary skill in the art in the field of solar cell design.

Even when it is known that particular variables have an impact on electrical, optical, chemical, thermal or other characteristics, the nature of the impact often cannot be predicted with much accuracy, particularly when the variables interact in complex ways, leading to unexpected results and unintended consequences. Thus, significant trial and error, which may include the fabrication and evaluative testing of many prototype devices, often over a period of time of months if not years, is required to determine whether a proposed structure with layers of particular compositions, actually will operate as intended, in a given environment over the operational life, let alone whether it can be fabricated in a reproducible high volume manner within the manufacturing tolerances and variability inherent in the production process, and necessary for the design of a commercially viable device.

Furthermore, as in the case here, where multiple variables interact in unpredictable ways, the proper choice of the combination of variables can produce new and “unexpected results”, and constitute an “inventive step” in designing and specifying a solar cell to operate in a predetermined environment (such as space), not only at the beginning of life, but over the entire defined operational lifetime.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

One aspect of the present disclosure relates to the use of aluminum in the active layers of the upper subcells in a multijunction solar cell. The effects of increasing amounts of aluminum as a constituent element in an active layer of a subcell affects the photovoltaic device performance. One measure of the “quality” or “goodness” of a solar cell subcell or junction is the difference between the band gap of the semiconductor material in that subcell or junction and the V_(oc), or open circuit voltage, of that same junction. The smaller the difference, the higher the V_(oc) of the solar cell junction relative to the band gap, and the better the performance of the device. V_(oc) is very sensitive to semiconductor material quality, so the smaller the E_(g)−V_(oc) of a device, the higher the quality of the material in that device. There is a theoretical limit to this difference, known as the Shockley-Queisser limit. That is the best that a solar cell junction can be under a given concentration of light at a given temperature.

The experimental data obtained for single junction (Al)GaInP solar cells indicates that increasing the Al content of the junction leads to a larger V_(oc)−E_(g) difference, indicating that the material quality of the junction decreases with increasing Al content. FIG. 1 shows this effect. The three compositions cited in the Figure are all lattice matched to GaAs, but have differing Al composition. As seen by the different compositions represented, with increasing amount of aluminum represented by the x-axis, adding more Al to the semiconductor composition increases the band gap of the junction, but in so doing also increases V_(oc)−E_(g). Hence, we draw the conclusion that adding Al to a semiconductor material degrades that material such that a solar cell device made out of that material does not perform relatively as well as a junction with less Al.

Turning to the fabrication of the multijunction solar cell assembly of the present disclosure, and in particular a five-junction solar cell assembly, FIG. 2A is a cross-sectional view of a first embodiment of a four junction solar cell subassembly 500 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate, and formation of grids and contacts on the contact layer of the semiconductor body.

As shown in the illustrated example of FIG. 2, the bottom subcell D₁ includes a substrate 600 formed of p-type germanium (“Ge”) in some embodiments, which also serves as a base layer. A back metal contact pad 650 formed on the bottom of base layer 600 provides electrical contact to the multijunction solar cell subassembly 500.

In some embodiments, the bottom subcell D₁ is germanium, while in other embodiments the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN. InGaSbBiN or other III-V or II-VI compound semiconductor material.

The bottom subcell D₁, further includes, for example, a highly doped n-type Ge emitter layer 601, and an n-type indium gallium arsenide (“InGaAs”) nucleation layer 602 a. The nucleation layer 602 a is deposited over the base layer, and the emitter layer 601 is formed in the substrate by diffusion of deposits into the Ge substrate, thereby forming the n-type Ge layer 601.

In the first solar cell subassembly 500 of FIG. 2, a highly doped lateral conduction layer 602 b is deposited over layer 601, and a blocking p-n diode or insulating layer 602 c is deposited over the layer 602 b.

In the embodiment of FIG. 2 of the present disclosure, an alpha layer 603, preferably composed of n-type AlGaInAsP, is deposited over the lateral conduction layer 602 d, to a thickness of between 0.25 and 1.0 micron. Such an alpha layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the bottom subcell D₁, or in the direction of growth into the subcell C₁, and is more particularly described in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.).

A metamorphic layer (or graded interlayer) 604 is deposited over the alpha layer 603 using a surfactant. Layer 604 is preferably a compositionally step-graded series of InGaAlAs layers, preferably with monotonically changing lattice constant, so as to achieve a gradual transition in lattice constant in the semiconductor structure from subcell D₁ to subcell C₁ while minimizing threading dislocations from occurring. In some embodiments, the band gap of layer 606 is not constant throughout its thickness, each sublayer having a band gap in the range equal to equal to 1.22 to 1.34 eV, and otherwise consistent with a value slightly greater than the band gap of the middle subcell C₁. One embodiment of the graded interlayer may also be expressed as being composed of In_(x)Ga_(1-x)As, with x and y selected such that the band gap of the interlayer is approximately 1.22 to 1.34 eV or other appropriate band gap.

In the surfactant assisted growth of the metamorphic layer 604, a suitable chemical element is introduced into the reactor during the growth of layer 604 to improve the surface characteristics of the layer. In the preferred embodiment, such element may be a dopant or donor atom such as selenium (Se) or tellurium (Te). Small amounts of Se or Te are therefore incorporated in the metamorphic layer 604, and remain in the finished solar cell. Although Se or Te are the preferred n-type dopant atoms, other non-isoelectronic surfactants may be used as well.

Surfactant assisted growth results in a much smoother or planarized surface. Since the surface topography affects the bulk properties of the semiconductor material as it grows and the layer becomes thicker, the use of the surfactants minimizes threading dislocations in the active regions, and therefore improves overall solar cell efficiency.

As an alternative to the use of non-isoelectronic one may use an isoelectronic surfactant. The term “isoelectronic” refers to surfactants such as antimony (Sb) or bismuth (Bi), since such elements have the same number of valence electrons as the P atom of InGaP, or the As atom in InGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactants will not typically be incorporated into the metamorphic layer 606.

In one embodiment of the present disclosure, the layer 604 is composed of a plurality of layers of InGaAs, with monotonically changing lattice constant, each layer having a band gap, approximately in the range of 1.22 to 1.34 eV. In some embodiments, the band gaps are in the range of 1.27 to 1.31 eV. In some embodiments, the band gaps are in the range of 1.28 to 1.29 eV.

The advantage of utilizing a constant bandgap material such as InGaAs is that arsenide-based semiconductor material is much easier to process in standard commercial MOCVD reactors.

Although the preferred embodiment of the present disclosure utilizes a plurality of layers of InGaAs for the metamorphic layer 604 for reasons of manufacturability and radiation transparency, other embodiments of the present disclosure may utilize different material systems to achieve a change in lattice constant from subcell C₁ to subcell D₁. Other embodiments of the present disclosure may utilize continuously graded, as opposed to step graded, materials. More generally, the graded interlayer may be composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater than or equal to that of the third solar cell and less than or equal to that of the fourth solar cell, and having a bandgap energy greater than that of the third solar cell.

An alpha layer 605, preferably composed of n+ type GaInP, is deposited over metamorphic buffer layer 604, to a thickness of about 1.0 micron. Such an alpha layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the subcell D₁, or in the direction of growth into the subcell C₁, and is more particularly described in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.).

Heavily doped p-type aluminum gallium arsenide (“AlGaAs”) and heavily doped n-type gallium arsenide (“GaAs”) tunneling junction layers 606, 607 may be deposited over the alpha layer 605 to provide a low resistance pathway between the bottom and middle subcells D₁ and C₁.

Distributed Bragg reflector (DBR) layers 608 are then grown adjacent to and between the alpha layer 605 and the third solar subcell C₁. The DBR layers 608 are arranged so that light can enter and pass through the third solar subcell C₁ and at least a portion of which can be reflected back into the third solar subcell C₁ by the DBR layers 608. In the embodiment depicted in FIG. 3, the distributed Bragg reflector (DBR) layers 608 are specifically located between the third solar subcell C and tunnel diode layer 607; in other embodiments, the distributed Bragg reflector (DBR) layers may be located between alpha layer 605 and tunnel diode layers 606/607.

For some embodiments, distributed Bragg reflector (DBR) layers 608 can be composed of a plurality of alternating layers 608 a through 608 z of lattice matched materials with discontinuities in their respective indices of refraction. For certain embodiments, the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength.

For some embodiments, distributed Bragg reflector (DBR) layers 608 a through 608 z includes a first DBR layer composed of a plurality of p type Al_(x)Ga_(1-x)(In)As layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type Al_(y)Ga_(1-y)(In)As layers, where y is greater than x.

On top of the DBR layers 608 subcell C₁ is grown.

In the illustrated example of FIG. 2, the subcell C₁ includes a highly doped p-type indium aluminum gallium arsenide (“InAlGaAs”) back surface field (“BSF”) layer 609, a p-type InGaAs base layer 610, a highly doped n-type indium gallium arsenide (“InGaAs”) emitter layer 611 and a highly doped n-type indium aluminum phosphide (“AlInP2”) window layer 612. The InGaAs base layer 610 of the subcell C₁ can include, for example, approximately 1.5% In. Other compositions may be used as well. The base layer 610 is formed over the BSF layer 609 after the BSF layer is deposited over the DBR layers 608 a through 608 z.

The window layer 612 is deposited on the emitter layer 611 of the subcell C₁. The window layer 612 in the subcell C₁ also helps reduce the recombination loss and improves passivation of the cell surface of the underlying junctions. Before depositing the layers of the subcell B, heavily doped n-type InGaP and p-type AlGaAs (or other suitable compositions) tunneling junction layers 613, 614 may be deposited over the subcell C₁.

The middle subcell B₁ includes a highly doped p-type indium aluminum gallium arsenide (“InAlGaAs”) back surface field (“BSF”) layer 615, a p-type AlGaAs base layer 616, a highly doped n-type indium gallium phosphide (“InGaP2”) or InAlGaAs layer 617 and a highly doped n-type indium gallium aluminum phosphide (“AlGaAlP”) window layer 618. The InGaP emitter layer 617 of the subcell B₁ can include, for example, approximately 50% In. Other compositions may be used as well.

Before depositing the layers of the top cell A₁, heavily doped n-type InGaP and p-type (In)AlGaAs tunneling junction layers 619, 620 may be deposited over the subcell B.

In the illustrated example, the top subcell A₁ includes a highly doped p-type indium aluminum phosphide (“InAlP”) BSF layer 621, a p-type InGaAlP base layer 622, a highly doped n-type InGaAlP emitter layer 623 and a highly doped n-type InAlP2 window layer 624. The base layer 623 of the top subcell A is deposited over the BSF layer 621 after the BSF layer 621 is formed over the tunneling junction layers 619, 620 of the subcell B₁. The window layer 624 is deposited over the emitter layer 623 of the top subcell A₁ after the emitter layer 623 is formed over the base layer 622.

A cap or contact layer 625 may be deposited and patterned into separate contact regions over the window layer 624 of the top subcell A₁. The cap or contact layer 625 serves as an electrical contact from the top subcell A₁ to metal grid layer 626. The doped cap or contact layer 625 can be a semiconductor layer such as, for example, a GaAs or InGaAs layer.

After the cap or contact layer 625 is deposited, the grid lines 626 are formed via evaporation and lithographically patterned and deposited over the cap or contact layer 625.

A contact pad 627 which is electrically connected to the grid line 626 is formed on one edge of the subassembly 500 to allow an electrical interconnection to be made to an adjacent subassembly.

Following deposition of the semiconductor layers, the semiconductor body 500 is etched so that several ledges or platforms are formed on intermediate layers so that electrical contact may be made thereto, in particular, in one embodiment, ledges 651, 661, and 671.

To this end, the solar cell subassembly can include a plurality of openings in the first semiconductor body, each of the openings extending from a top surface of the first semiconductor body to a different respective contact layer (651, 661, 671) in the first semiconductor body. Such “openings” may include recesses, cavities, holes, gaps, cut-outs, or similar structures, but for simplicity we will subsequently just use the term “opening” throughout this disclosure. In other implementations, we can etch through the rear of the substrate and have all the openings come from the back side. This approach is more efficient as it does not shadow the top two or top three solar subcells, but it results in a solar epitaxial structure of only a few 10 s of microns in thickness.

A metal contact pad 672 is deposited on the surface of the ledge 671 which exposes a portion of the top surface of the metamorphic layer 604. This pad 672 allows electrical contact to be made to the bottom of the stack of subcells A₁ through C₁.

A metal contact pad 602 f is deposited on the surface of the ledge of 661 which exposes a portion of the top surface of the lateral conduction layer 602 b. This pad 602 f allows electrical contact to be made to the top or n-terminal of the subcell D.

A metal contact 651 is further provided as part of the back metal layer 650 which allows electrical contact to be made to the p-terminal of subcell D.

FIG. 3 illustrates a second solar cell subassembly 700, which is similar to the solar cell subassembly 500 of FIG. 2 in that it has substantially the same sequence of semiconductor layers with the same compositions and band gaps as the corresponding layers in the first solar cell subassembly 500. Thus, the solar cell subassembly 700 also includes multiple subcells in a tandem stack. In the illustrated example of FIG. 3, the second solar cell subassembly 700 includes an upper first subcell (Subcell A₂), a second and third solar subcells (Subcell B₂ and C₂) disposed adjacent to and lattice matched to the upper first subcell A₂, and a bottom subcell (Subcell E) lattice mismatched to the third subcell C₂.

As with the first solar cell subassembly 500, the subcells A₂, B₂, C₂ of the second solar cell subassembly can be configured so that the short circuit current densities of the three subcells A₂, B₂, C₂ have a substantially equal predetermined first value (J1=J2=J3), and the short circuit current density (J4) of the bottom subcell E is at least twice that of the predetermined first value.

Since the semiconductor layers 700 through 725 in subassembly 700 are substantially identical to layers 600 through 625 in subassembly 500, a detailed description of them will not be provided here for brevity.

In order to provide access to the various layers in the second solar cell subassembly 700, various ones of the layers can be exposed partially. Thus, as shown in the example of FIG. 3, various surfaces are partially exposed on the left side of the subassembly 700, for example, using standard photolithographic etching techniques to etch from the top surface of the semiconductor body 700 to the particular contact layer 771, 761 and 751 of interest.

A metal contact pad 772 is deposited on the surface of the ledge of 771 which exposes a portion of the top surface of the metamorphic layer 704. This pad 772 allows electrical contact to be made to the bottom of the stack of subcells A₂ through C₂.

A metal contact pad 762 is deposited on the surface of the ledge of 761 which exposes a portion of the top surface of the lateral conduction layer 702 b. This pad 762 allows electrical contact to be made to the top or n-terminal of the subcell E.

A metal contact 751 is further provided as part of the back metal layer 750 which allows electrical contact to be made to the p-terminal of subcell E.

A second embodiment of the second solar cell subassembly (not illustrated) similar to that of FIG. 3 is another configuration with that the metamorphic buffer layer 604 is disposed above the tunnel diode layers 706, 707 and below the DBR layers 708 (not illustrated).

The foregoing multijunction solar cell subassemblies 500, 600, or 700 can be fabricated, for example, in wafer-level processes and then diced into individual semiconductor chips. The various semiconductor layers can be grown, one atop another, using known growth techniques (e.g., MOCVD) as discussed above.

Each solar cell subassembly 500, 600, 700 also can include grid lines, interconnecting bus lines, and contact pads. The geometry and number of the grid lines, bus lines and/or contacts may vary in other implementations.

As previously mentioned, two (or more) solar cell subasemblies (e.g., 500 and 700) can be connected together electrically. For example, as shown in FIG. 4, conductive (e.g., metal) interconnections 801, 802, 803, and 805 can be made between different layers of the solar cell subassemblies 500, 700. Some of the interconnections are made between different layers of a single one of the solar cell subassemblies, whereas others of the interconnections are made between the two different solar cell subassemblies. Thus, for example, the interconnection 801 electrically connects together the metal contacts 627 and 727 of the first and second solar cell subassemblies 500 and 700 respectively. In particular, interconnection 803 connects together a contact 672 on the metamorphic layer 604 of the first solar cell subassembly 500 to a contact 772 on the metamorphic layer 704 of the second solar cell subassembly 700. Similarly, the interconnection 805 connects together a contact 651 on the back metal layer 650 of the first solar cell subassembly 500 to a contact 762 on the lateral conduction layer 702 b of the second solar cell subassembly 700. Likewise, the interconnection 802 connects together a contact 672 on the metamorphic layer 604 of the first solar cell subassembly 500 to a contact 602 f on the lateral conduction layer 602 b of the first solar cell subassembly 500.

In some instances, multiple electrically conductive (e.g., metal) contacts can be provided for each of the respective contacts of the solar cell subassemblies 500, 700. This allows each of the interconnections 801-804 to be implemented by multiple interconnections between the solar cell subassembly layers rather than just a single interconnection.

As noted above, the solar cell assembly includes a first electrical contact of a first polarity and a second electrical contact of a second polarity. In some embodiments, the first electrical contact 807 is connected to the metal contact 627 on the first solar cell subassembly 500 by an interconnection 806, and the second electrical contact 808 is connected to the back metal contact 751 of the second solar cell subassembly 700 by an interconnection 809.

As illustrated in FIG. 5, two or more solar cell subassemblies can be connected electrically as described above to obtain a multijunction (e.g. a four-, five- or six-junction) solar cell assembly. In FIG. 5, the top side (n-polarity) conductivity contact 807 and bottom side (p-polarity) conductive contact 808 for the solar cell assembly are schematically depicted respectively, at the left and right-hand sides of the assembly.

In the example of FIG. 5, one solar cell subassembly 500 includes an upper subcell A₁, two middle subcells B₁, C₁ and a bottom subcell D. The other solar cell subassembly 700 includes an upper subcell A₂, two middle subcells B₂, C₂ and a bottom subcell E. In some implementations, each solar cell subassembly 500, 700 has band gaps in the range of 2.0 to 2.20 eV (or higher) for the top subcell, and (i) 1.65 to 1.8, and (ii) 1.41 eV for the middle subcells, and 0.6 to 0.8 eV, for the bottom subcell, respectively, Further, in some embodiments, the average band gap of all four subcells (i.e., the sum of the four band gaps of each subcell divided by four) in a given solar cell subassembly 500 or 700 is greater than 1.44 eV. Other band gap ranges may be appropriate for some implementations.

In some instances, the fourth (i.e., bottom) subcell is composed of germanium. The indirect band gap of the germanium at room temperature is about 0.66 eV, while the direct band gap of germanium at room temperature is 0.8 eV. Those skilled in the art with normally refer to the “band gap” of germanium as 0.66 eV, since it is lower than the direct band gap value of 0.8 eV. Thus, in some implementations, the fourth subcell has a direct band gap of greater than 0.75 eV. Reference to the fourth subcell having a direct band gap of greater than 0.75 eV is expressly meant to include germanium as a possible semiconductor material for the fourth subcell, although other semiconductor materials can be used as well. For example, the fourth subcell may be composed of InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, or other III-V or II-VI compound semiconductor materials.

In some implementations of a five-junction solar cell assembly, such as in the example of FIG. 5, the short circuit density (J_(sc)) of the upper first subcells (A₁, and A₂) and the middle subcells (B₁, B₂, C₁, C₂) is about 12 mA/cm², and the short circuit current density (J_(sc)) of the bottom subcells (D and E) is about 24 mA/cm². Other implementations may have different values.

The present disclosure, like related application Ser. No. 14/828,206, provides a multijunction solar cell that follows a design rule that one should incorporate as many high band gap subcells as possible to achieve the goal to increase the efficiency at either low temperature, room temperature (28° C.), or high temperature (50 to 70° C.) EOL. For example, high band gap subcells may retain a greater percentage of cell voltage as temperature increases, thereby offering lower power loss as temperature increases. As a result, both HT-BOL and HT-EOL performance of the exemplary multijunction solar cell, according to the present disclosure, may be expected to be greater than traditional cells.

In view of different satellite and space vehicle requirements in terms of temperature, radiation exposure, and operational life, a range of subcell designs using the design principles of the present disclosure may be provided satisfying typical customer and mission requirements, and several embodiments are set forth hereunder, along with the computation of their efficiency at the end-of-life. The radiation exposure is experimentally measured using 1 MeV electron fluence per square centimeter (abbreviated in the text that follows as e/cm²), so that a comparison can be made between the current commercial devices and embodiments of solar cells discussed in the present disclosure.

As an example, a low earth orbit (LEO) satellite will typically experience radiation equivalent to 5×10¹⁴ e/cm² over a five year lifetime. A geosynchronous earth orbit (GEO) satellite will typically experience radiation in the range of 5×10¹⁴ e/cm² to 1×10 e/cm² over a fifteen year lifetime.

As a baseline for comparison, the cell efficiency (%) measured at room temperature (RT) 28° C. and high temperature (HT) 70° C., at beginning of life (BOL) and end of life (EOL), for a standard three junction commercial solar cell (e.g. a SolAero Technologies Corp. Model ZTJ), such as depicted in FIG. 2 of U.S. patent application Ser. No. 14/828,206, is as follows:

Condition Efficiency BOL 28° C. 29.1% BOL 70° C. 26.4% EOL 70° C. 23.4% After 5E14 e/cm² radiation EOL 70° C. 22.0% After 1E15 e/cm² radiation

For the 5J solar cell assembly described in the present disclosure, the corresponding data is as follows:

Condition Efficiency BOL 28° C. 30.6% BOL 70° C. 27.8% EOL 70° C. 26.6% After 5E14 e/cm² radiation EOL 70° C. 26.1% After 1E15 e/cm² radiation

The new solar cell described in the present disclosure has a slightly higher cell efficiency than the standard commercial solar cell (ZTJ) at BOL at 70° C. However, the solar cell described in the present disclosure exhibits substantially improved cell efficiency (%) over the standard commercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of 5×10¹⁴ e/cm², and dramatically improved cell efficiency (%) over the standard commercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of 1×10¹⁵ e/cm².

The wide range of electron and proton energies present in the space environment necessitates a method of describing the effects of various types of radiation in terms of a radiation environment which can be produced under laboratory conditions. The methods for estimating solar cell degradation in space are based on the techniques described by Brown et al. [Brown, W. L., J. D. Gabbe, and W. Rosenzweig, Results of the Telstar Radiation Experiments, Bell System Technical J., 42, 1505, 1963] and Tada [Tada, H. Y., J. R. Carter, Jr., B. E. Anspaugh, and R. G. Downing, Solar Cell Radiation Handbook, Third Edition, JPL Publication 82-69, 1982]. In summary, the omnidirectional space radiation is converted to a damage equivalent unidirectional fluence at a normalised energy and in terms of a specific radiation particle. This equivalent fluence will produce the same damage as that produced by omnidirectional space radiation considered when the relative damage coefficient (RDC) is properly defined to allow the conversion. The relative damage coefficients (RDCs) of a particular solar cell structure are measured a priori under many energy and fluence levels. When the equivalent fluence is determined for a given space environment, the parameter degradation can be evaluated in the laboratory by irradiating the solar cell with the calculated fluence level of unidirectional normally incident flux. The equivalent fluence is normally expressed in terms of 1 MeV electrons or 10 MeV protons.

The software package Spenvis (www.spenvis.oma.be) is used to calculate the specific electron and proton fluence that a solar cell is exposed to during a specific satellite mission as defined by the duration, altitude, azimuth, etc. Spenvis employs the EQFLUX program, developed by the Jet Propulsion Laboratory (JPL) to calculate 1 MeV and 10 MeV damage equivalent electron and proton fluences, respectively, for exposure to the fluences predicted by the trapped radiation and solar proton models for a specified mission environment duration. The conversion to damage equivalent fluences is based on the relative damage coefficients determined for multijunction cells [Marvin, D. C., Assessment of Multijunction Solar Cell Performance in Radiation Environments, Aerospace Report No. TOR-2000 (1210)-1, 2000]. A widely accepted total mission equivalent fluence for a geosynchronous satellite mission of 15 year duration is 1 MeV 1×10¹⁵ electrons/cm².

The exemplary solar cell described herein may require the use of aluminum in the semiconductor composition of each of the top two subcells. Aluminum incorporation is widely known in the III-V compound semiconductor industry to degrade BOL subcell performance due to deep level donor defects, higher doping compensation, shorter minority carrier lifetimes, and lower cell voltage and an increased BOL E_(g)−V_(oc) metric. In short, increased BOL E_(g)−V_(oc) may be the most problematic shortcoming of aluminum containing subcells; the other limitations can be mitigated by modifying the doping schedule or thinning base thicknesses.

It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of structures or constructions differing from the types of structures or constructions described above.

Although described embodiments of the present disclosure utilizes a vertical tandem stack of four subcells, various aspects and features of the present disclosure can apply to tandem stacks with fewer or greater number of subcells, i.e. two junction cells, three junction cells, five junction cells, etc.

In addition, although the disclosed embodiments are configured with top and bottom electrical contacts, the subcells may alternatively be contacted by means of metal contacts to laterally conductive semiconductor layers between the subcells. Such arrangements may be used to form 3-terminal, 4-terminal, and in general, n-terminal devices. The subcells can be interconnected in circuits using these additional terminals such that most of the available photogenerated current density in each subcell can be used effectively, leading to high efficiency for the multijunction cell, notwithstanding that the photogenerated current densities are typically different in the various subcells.

As noted above, the solar cell described in the present disclosure may utilize an arrangement of one or more, or all, homojunction cells or subcells, i.e., a cell or subcell in which the p-n junction is formed between a p-type semiconductor and an n-type semiconductor both of which have the same chemical composition and the same band gap, differing only in the dopant species and types, and one or more heterojunction cells or subcells. Subcell A, with p-type and n+ type InGaAlP is one example of a homojunction subcell.

In some cells, a thin so-called “intrinsic layer” may be placed between the emitter layer and base layer, with the same or different composition from either the emitter or the base layer. The intrinsic layer may function to suppress minority-carrier recombination in the space-charge region. Similarly, either the base layer or the emitter layer may also be intrinsic or not-intentionally-doped (“NID”) over part or all of its thickness.

The composition of the window or BSF layers may utilize other semiconductor compounds, subject to lattice constant and band gap requirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AIN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fall within the spirit of the present invention.

While the solar cell described in the present disclosure has been illustrated and described as embodied in a conventional multijunction solar cell, it is not intended to be limited to the details shown, since it is also applicable to inverted metamorphic solar cells, and various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Thus, while the description of the semiconductor device described in the present disclosure has focused primarily on solar cells or photovoltaic devices, persons skilled in the art know that other optoelectronic devices, such as thermophotovoltaic (TPV) cells, photodetectors and light-emitting diodes (LEDS), are very similar in structure, physics, and materials to photovoltaic devices with some minor variations in doping and the minority carrier lifetime. For example, photodetectors can be the same materials and structures as the photovoltaic devices described above, but perhaps more lightly-doped for sensitivity rather than power production. On the other hand LEDs can also be made with similar structures and materials, but perhaps more heavily-doped to shorten recombination time, thus radiative lifetime to produce light instead of power. Therefore, this invention also applies to photodetectors and LEDs with structures, compositions of matter, articles of manufacture, and improvements as described above for photovoltaic cells.

Without further analysis, from the foregoing others can, by applying current knowledge, readily adapt the present invention for various applications. Such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims. 

1. A multijunction solar cell including a terminal of first polarity and a terminal of second polarity, comprising: (a) a semiconductor body including a tandem vertical stack of epitaxial subcells including at least an upper solar subcell, a first middle solar subcell, and a bottom solar subcell, the upper subcell having a top contact connected to the terminal of the first polarity, and the bottom solar subcell having a top contact and a bottom contact; (b) an epitaxial first lateral conduction interlayer disposed adjacent to and beneath the second solar subcell to provide an electrical contact to the bottom of the second solar subcell to provide an electrical contact to the bottom of the second solar subcell; (c) an epitaxial blocking p-n diode or insulating layer disposed adjacent to and beneath the first lateral conduction interlayer; and (d) an epitaxial second lateral conduction layer disposed adjacent to and beneath the blocking p-n diode or insulating layer, wherein the bottom solar subcell is disposed adjacent to and beneath the second lateral conduction layer.
 2. A solar cell as defined in claim 1, wherein the first lateral conduction layer is compositionally graded to substantially lattice match the first middle solar subcell on one side and the bottom solar subcell on the other side, and is composed of any As, P, N, Sb based III-V compound semiconductors subject to constraints of having an in-plane lattice parameter less than or equal to that of the second solar subcell and greater than or equal to that of the bottom solar cell of the respective semiconductor region.
 3. A solar cell as defined in claim 1, further comprising: a first electrical contact disposed on the first lateral conduction layer and a second electrical contact disposed on the second lateral conduction layer.
 4. A solar cell as defined in claim 1, wherein the respective tandem vertical stack includes a second middle solar subcell, so that at least a four junction solar cell is formed, wherein the respective upper and first and second middle solar subcells are current mismatched from the bottom solar subcell.
 5. A solar cell as defined in claim 4, wherein the bottom subcell has a band gap of approximately 0.67 eV, the second middle subcell has a band gap in the range of approximately 1.3 to 1.5 eV, the first middle solar subcell has a band gap in the range of 1.65 to 1.8 eV, and the upper subcell has a band gap in the range of 1.8 to 2.0 eV.
 6. A solar cell as defined in claim 4, wherein the bottom solar subcell has a band gap of approximately 0.67 eV, the second middle solar subcell has a band gap in the range of approximately 1.41 eV and 1.31 eV, the first middle solar subcell has a band gap in the range of approximately 1.65 to 1.8 eV and the upper solar subcell of the first and second semiconductor regions has a band gap in the range of 2.0 to 2.20 eV.
 7. A solar cell as defined in claim 4, wherein the first lateral conduction interlayer is composed of In_(x)Al_(y)Ga_(1-x-y)As or In_(x)Ga_(1-x)P with 0<x<1, 0<y<1, and x and y selected such that the band gap is in the range of 1.41 eV to 1.6 eV and may vary throughout its thickness, and is compositionally graded to substantially lattice match the second solar subcell on one side and the bottom solar subcell on the other side.
 8. A solar cell as defined in claim 4, wherein the upper subcell is composed of indium gallium aluminum phosphide (InGaAlP); the second middle solar subcell includes an emitter layer composed of indium gallium phosphide (InGaP), indium aluminum gallium arsenide (InAlGaAs) or indium gallium arsenide phosphide (InGaAsP), and a base layer composed of indium aluminum gallium arsenide or indium gallium arsenide phosphide (InGaAsP); the first middle solar subcell is composed of indium gallium arsenide; the fourth subcell is composed of germanium or SiGeSn, GaSb, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN; and the first and second lateral conduction interlayers are composed of In_(x)Al_(y)Ga_(1-x-y)As or In_(x)Ga_(1-x)P with 0<x<1, 0<y<1, and x and y selected such that the band gap is in the range of 1.41 eV to 1.6 eV and may vary throughout its thickness.
 9. A solar cell as defined in claim 4, wherein: the upper subcell has a band gap in the range of 2.0 to 2.20 eV and is composed of indium gallium aluminum phosphide (InGaAlP); the second middle solar subcell has a band gap in the range of approximately 1.65 to 1.8 eV and includes an emitter layer composed of indium gallium phosphide (InGaP), indium aluminum gallium arsenide (InGaAs) or indium gallium arsenide phosphide (InGaAsP), and a base layer composed of indium aluminum gallium arsenide or indium gallium arsenide phosphide (InGaAsP); the first middle solar subcell has a band gap in the range of approximately 1.3 to 1.41 eV and is composed of indium gallium arsenide; and the bottom solar subcell is composed of germanium or SiGeSn, GaSb, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN, InGaSbBiN; the upper first subcell is composed of indium gallium aluminum phosphide.
 10. A solar cell as defined in claim 1, further comprising: a distributed Bragg reflector (DBR) layer disposed above the bottom solar subcell and arranged so that light can enter and pass through the solar subcell disposed above the bottom subcell and at least a portion of which can be reflected back into the solar subcell located above the bottom subcell by the DBR layer, and the distributed Bragg reflector layer is composed of a plurality of alternating layers of lattice matched materials with discontinuities in their respective indices of refraction.
 11. A solar cell as defined in claim 10, wherein the thickness and refractive index of each period determines the stop band and its limiting wavelength, and the DBR layer includes a first DBR layer composed of a plurality of n or p type Al_(x)Ga_(1-x)(In)As layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of n or p type Al_(y)Ga_(1-y)(In)As layers, where 0<x<1, 0<y<1, and y is greater than x, and the term (In) denotes an optional inclusion of up to 10% indium.
 12. A solar cell as defined in claim 4, wherein the selection of the composition of the subcells and their band gaps maximizes the efficiency at high temperature (in the range of 40 to 100 degrees Centigrade) in deployment in space at a predetermined time after the initial deployment (referred to as the beginning-of-life or BOL), such predetermined time being referred to as the end-of-life (EOL), and the average band gap of all four cells (i.e., the sum of the four lowest direct or indirect band gaps of the materials of each subcell divided by 4) is greater than 1.44 eV.
 13. A solar cell as defined in claim 4, wherein the bottom solar subcell is comprised of a direct or indirect band gap material such that the lowest direct band gap of the material is greater than 0.75 eV, and the average band gap of the upper solar subcell, the first middle solar subcell, the second middle solar subcell and the bottom solar subcell is greater than 1.44 eV.
 14. A solar cell as defined in claim 1, further comprising a tunnel diode layer over the first lateral conduction layer.
 15. A solar cell as defined in claim 1, further comprising a first alpha layer deposited over the first lateral conduction layer of the respective semiconductor region to a thickness of between 0.25 and 1.0 micron to prevent threading dislocations from propagating, either opposite to the direction of growth or in the direction of growth into the first middle solar subcell.
 16. A solar cell as defined in claim 15, further comprising a second alpha layer deposited over the blocking p-n diode or insulating layer to a thickness of between 0.25 and 1.0 micron to prevent threading dislocations from propagating, either opposite to the direction of growth or in the direction of growth into the second solar subcell.
 17. A solar cell as defined in claim 10, further comprising a tunnel diode disposed over the first lateral conduction layer and below the distributed Bragg reflector (DBR) layer.
 18. A solar cell as defined in claim 1, wherein the bottom contact of the bottom solar subcell is connected to the terminal of second polarity.
 19. A multijunction solar cell including a terminal of first polarity and a terminal of second polarity, comprising: (a) a semiconductor body including: a tandem vertical stack of at least an epitaxial upper solar subcell, first middle solar subcell, second middle solar subcell and bottom solar subcell, the upper subcell having a top contact connected to the terminal of the first polarity, and the bottom solar subcell having a top contact and a bottom contact connected to the terminal of second polarity; (b) an epitaxially deposited first lateral conduction interlayer disposed adjacent to and beneath the second solar subcell to provide an electrical contact to the bottom of the second solar sub cell; (c) an epitaxially deposited blocking p-n diode or insulating layer disposed adjacent to and beneath the first lateral conduction interlayer; and (d) an epitaxially deposited second lateral conduction layer disposed adjacent to and beneath the blocking p-n diode or insulating layer, wherein the bottom solar subcell is disposed adjacent to and beneath the second lateral conduction layer which forms an electrical contact to the top contact of the bottom solar subcell.
 20. A method of forming a multijunction solar cell assembly comprising: (a) providing an integral semiconductor body including a tandem vertical stack of epitaxially grown subcells including at least an upper solar subcell, a first middle solar subcell, a second middle solar subcell and a bottom solar subcell, the upper subcell having a top contact connected to a terminal of the first polarity, and the bottom solar subcell having a bottom contact; an epitaxially deposited first lateral conduction layer disposed adjacent to and beneath the second solar subcell thereby forming a bottom contact to the first middle solar subcell; an epitaxially deposited blocking p-n diode or insulating layer disposed adjacent to and beneath the first lateral conduction interlayer of the respective semiconductor region; an epitaxially deposited second lateral conduction layer disposed adjacent to and beneath the blocking p-n diode or insulating layer, wherein the bottom solar subcell is disposed adjacent to and beneath the second lateral conduction layer; (b) etching from the top surface of the semiconductor body to the first lateral conduction layer to make an electrical contact to the first lateral conduction layer and thereby to the bottom contact to the first middle solar subcell; and (c) etching from the top surface of the semiconductor body to the second lateral conduction layer to make an electrical contact to the second lateral conduction layer and thereby to the top contact of the bottom solar subcell. 